Medical device

ABSTRACT

A method for treating a bifurcation or trifurcation aneurysm ( 201 ) occurring on a first artery, the first artery and a second artery joining to a third artery, the method comprising: inserting a medical device ( 202 ) such that it is at least partially located in the first artery and is at least partially located in the third artery; expanding the medical device ( 202 ) from a first position to a second position, said medical device ( 202 ) is expanded radially outwardly to the second position such that the exterior surface of said medical device ( 202 ) engages with the inner surface of the first and third arteries so as to maintain a fluid pathway through said arteries; and positioning the medical device ( 202 ) such that a membrane ( 203 ) of the medical device ( 202 ) is located against an aneurysm neck of the aneurysm ( 201 ) to obstruct blood circulation to the aneurysm ( 201 ) when the medical device ( 202 ) is expanded to the second position, and at least a portion of the membrane ( 203 ) is secured to the medical device ( 202 ) to maintain the position of the membrane ( 203 ) relative to the medical device ( 202 ) when expanded to the second position; wherein the membrane ( 203 ) is permeable and porous, the size of the pores of the membrane ( 203 ) and the ratio of the material surface area of the membrane ( 203 ) being such that blood supply to perforators and/or microscopic branches of main brain arteries is permitted to improve healing of the first artery but blood supply to the aneurysm ( 201 ) is prevented.

CROSS REFERENCES TO RELATED APPLICATIONS

This is a continuation-in-part application from U.S. patent applicationSer. No. 10/580,139 filed on May 19, 2006, which claims priority fromInternational Patent Application No. PCT/SG2004/000407 filed on Dec. 13,2004, which claims priority from Singapore Patent Application No.200401735-6 filed on Mar. 31, 2004.

FIELD OF THE INVENTION

The present invention relates to a medical device for insertion into abodily vessel to treat an aneurysm.

BACKGROUND OF THE INVENTION

Vascular diseases include aneurysms causing hemorrhage, atherosclerosiscausing the occlusion of blood vessels, vascular malformation andtumors. Vessel occlusion or rupture of an aneurysm within the braincauses of stroke. Aneurysms fed by intracranial arteries can grow withinthe brain to a point where their mass and size can cause a stroke or thesymptoms of stroke, requiring surgery for removal of the aneurysms orother remedial intervention.

Occlusion of coronary arteries, for example, is a common cause of heartattack. Diseased and obstructed coronary arteries can restrict the flowof blood in the heart and cause tissue ischemia and necrosis. While theexact etiology of sclerotic cardiovascular disease is still in question,the treatment of narrowed coronary arteries is more defined. Surgicalconstruction of coronary artery bypass grafts (CABG) is often the methodof choice when there are several diseased segments in one or multiplearteries. Conventional open-heart surgery is, of course, very invasiveand traumatic for patients undergoing such treatment. Therefore,alternative methods being less traumatic are highly desirable.

One of the alternative methods is balloon angioplasty that is atechnique in which a folded balloon is inserted into a stenosis, whichoccludes or partially occludes an artery and is inflated to open theoccluded artery. Another alternative method is atherectomy that is atechnique in which occlusive atheromas are cut from the inner surface ofthe arteries. Both methods suffer from reocclusion with certainpercentage of patients.

A recent preferred therapy for vascular occlusions is placement of anexpandable metal wire-frame including a stent, within the occludedregion of blood vessel to hold it open. The stent is delivered to thedesired location within a vascular system by a delivery means, usually acatheter. Advantages of the stent placement method over conventionalvascular surgery include obviating the need for surgically exposing,removing, replacing, or by-passing the defective blood vessel, includingheart-lung by-pass, opening the chest, and general anaesthesia.

When inserted and deployed in a vessel, duct or tract (“vessel”) of thebody, for example, a coronary artery after dilatation of the artery byballoon angioplasty, a stent acts as a prosthesis to maintain the vesselopen. The stent usually has an open-ended tubular form withinterconnected struts as its sidewall to enable its expansion from afirst outside diameter which is sufficiently small to allow the stent totraverse the vessel to reach a site where it is to be deployed, to asecond outside diameter sufficiently large to engage the inner lining ofthe vessel for retention at the site. A stent is typically delivered inan unexpanded state to a desired location in a body lumen and thenexpanded. The stent is expanded via the use of a mechanical device suchas a balloon, or the stent is self-expanding.

Usually a suitable stent for successful interventional placement shouldpossess features of relatively non-allergenic reaction, goodradiopacity, freedom from distortion on magnetic resonance imaging(MRI), flexibility with suitable elasticity to be plasticallydeformable, strong resistance to vessel recoil, sufficient thinness tominimize obstruction to flow of blood (or other fluid or material invessels other than the cardiovascular system), and biocompatibility toavoid of vessel re-occlusion. Selection of the material of which a stentis composed, as well as design of the stent, plays an important role ininfluencing these features.

Furthermore, implantable medical devices have been utilized for deliveryof drugs or bioreagents for different biological applications.Typically, the drugs or bioreagents are coated onto the surfaces of theimplantable medical devices or mixed within polymeric materials that arecoated onto the surfaces of the implantable medical devices. However,all the current available methods suffer from one or more problemsincluding uncontrollable release, form limitations of drugs, and bulkyappearance.

Therefore, there is desire for an implantable medical device that isable to deliver drugs or reagents efficiently to the endovascularsystem, especially intracranial blood vessels.

A method for treating bifurcation and trifurcation aneurysms isdisclosed in the previously filed cross-related application entitled “AMethod for Treating Aneurysms”, the contents of which are hereinincorporated by reference.

SUMMARY OF THE INVENTION

In a first preferred aspect, there is provided a method for treating abifurcation or trifurcation aneurysm occurring on a first artery, thefirst artery and a second artery joining to a third artery, the methodcomprising:

-   -   inserting a medical device such that it is at least partially        located in the first artery and is at least partially located in        the third artery;    -   expanding the medical device from a first position to a second        position, said medical device is expanded radially outwardly to        the second position such that the exterior surface of said        medical device engages with the inner surface of the first and        third arteries so as to maintain a fluid pathway through said        arteries; and    -   positioning the medical device such that a membrane of the        medical device is located against an aneurysm neck of the        aneurysm to obstruct blood circulation to the aneurysm when the        medical device is expanded to the second position, and at least        a portion of the membrane is secured to the medical device to        maintain the position of the membrane relative to the medical        device when expanded to the second position;    -   wherein the membrane is permeable and porous, the size of the        pores of the membrane and the ratio of the material surface area        of the membrane being such that blood supply to perforators        and/or microscopic branches of main brain arteries is permitted        to improve healing of the first artery but blood supply to the        aneurysm is prevented.

The medical device may be inserted such that blood circulation to thesecond artery is unobstructed by the membrane.

The distance between adjacent pores may be from about 40 to 100 microns.

The membrane may be made of a biocompatible and elastomeric polymer.

The membrane may have a thickness of about 0.0005 to 0.005″.

The ratio of the material surface area of the membrane may be from about25 to 75%.

The membrane may have pores between 20 to 100 microns in size.

The membrane may be made from polymeric material or biodegradablematerial.

The biodegradable material may form multiple sub-layers mixed with drugsor reagents.

The at least one reagent may be any one form selected from the groupconsisting of: solid tablet, liquid and powder.

The membrane may be capable of isotropic expansion.

The membrane may be disposed on the exterior surface of the device.

The membrane may circumferentially surround a portion of the device.

The membrane may cover a portion of the device.

The membrane may have fabricated pores between 20 to 100 microns insize.

The pores may be fabricated by laser drilling.

The distance between the pores may be less than 100 μm.

The membrane may comprise a plurality of polymeric strips secured to themedical device.

The strips may be less than 0.075 mm and the distance between adjacentstrips is less than 100 μm.

The membrane may comprise a mesh secured to the medical device.

Spaces of the mesh may be less than 100 μm and the width of the meshingis between 0.025 to 0.050 mm.

The aneurysm may be any one from the group consisting of: a regularsize, giant or wide neck aneurysm having an aneurysm neck greater than 4millimeters or a dome to neck ratio greater than 2, berry aneurysm, CCfistula and fusiform aneurysm.

The medical device may comprise a generally tubular structure having anexterior surface defined by a plurality of interconnected struts havinginterstitial spaces therebetween.

The medical device may be self-expandable or balloon expandable.

The membrane may be supported by the generally tubular structure and isattached to at least one strut.

The medical device may be a stent.

The membrane may be tubular having a diameter similar to a nominalinitial diameter of the stent; and wherein the membrane is disposed ontothe outer surface of the stent or introduced by dip coating or sprayingbetween the struts of the stent.

The membrane may be a segment of a tubular structure disposed onto aportion of the outer surface of the stent.

The membrane may substantially cover the entire circumferential surfaceof the medical device.

The permeability and porosity of the membrane may alter the hemodynamicsof the aneurysm sac of the aneurysm to initiate intra-aneurysmalthrombosis.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of the invention will now be described with reference to theaccompanying drawings, in which:

FIGS. 1A and 1B are two exemplary balloon expandable stents;

FIG. 2 shows a self-expanding stent;

FIG. 3A is diagrammatic view of a stent disposed in the location of ananeurysm;

FIG. 3B is diagrammatic view as FIG. 3A except that a port of the stentis formed of opened cells;

FIG. 4 shows a delivery system with a stent expanded onto the balloon;

FIG. 5 is diagrammatic view of a stent partially covered by a membranewith pockets;

FIG. 6 is a cross-sectional view of a sleeve as a membrane supported bytwo ring-like stents;

FIG. 7 is a diagrammatic view of a membrane joining two stents fortreating a bifurcation aneurysm;

FIG. 8 is a diagrammatic view of an aneurysm covered with the membraneof a stent to obstruct blood circulation to the aneurysm;

FIG. 9 is a table of typical dimensions for the stent;

FIG. 10 is a diagrammatic view of a stent with a membrane having apattern of pores;

FIG. 11 is a diagrammatic view of a stent with a membrane having polymerstrips;

FIG. 12 is a diagrammatic view of a stent with a membrane having a mesh;

FIG. 13 is a diagrammatic view of a membrane secured to the struts of astent;

FIG. 14 is a diagrammatic view of a membrane before the stent isdeployed;

FIG. 15 is a diagrammatic view of a stent with a membrane secured atthree different positions and with three different sizes;

FIG. 16 is a diagrammatic view of a membrane flipping inside the vesselrather than staying close the vessel wall;

FIG. 17 is a diagrammatic view of a stent with a membrane being used totreat a bifurcation aneurysm in a first example;

FIG. 18 is a diagrammatic view of a stent with a membrane being used totreat a bifurcation aneurysm in a second example; and

FIG. 19 is a diagrammatic view of a stent with a membrane being used totreat a bifurcation aneurysm in a third example.

DETAILED DESCRIPTION OF THE DRAWINGS

Implantable medical devices include physical structures for deliveringdrugs or reagents to desired sites within the endovascular system of ahuman body. Implantable medical devices may take up diversified shapesand configurations depending upon specific applications. Commonimplantable medical devices include stents, vena cava filters, graftsand aneurysm coils. While stents are described, it is noted that thedisclosed structures and methods are applicable to all the otherimplantable medical devices.

The endovascular system of a human body includes blood vessels, cerebralcirculation system, tracheo-bronchial system, the biliary hepaticsystem, the esophageal bowel system, and the urinary tract system.Although exemplary stents implantable 202 in blood vessels aredescribed, they are applicable to the remaining endovascular system.

Stents 202 are expandable prostheses employed to maintain vascular andendoluminal ducts or tracts of the human body open and unoccluded, suchas a portion of the lumen of a coronary artery after dilatation of theartery by balloon angioplasty. A typical stent 202 is a generallytubular structure having an exterior surface defined by a plurality ofinterconnected struts having interstitial spaces there between. Thegenerally tubular structure is expandable from a first position, whereinthe stent is sized for intravascular insertion, to a second position,wherein at least a portion of the exterior surface of the stent contactsthe vessel wall. The expanding of the stent is accommodated by flexingand bending of the interconnected struts throughout the generallytubular structure. It is contemplated that many different stent designscan be produced. A myriad of strut patterns are known for achievingvarious design goals such as enhancing strength, maximizing theexpansion ratio or coverage area, enhancing longitudinal flexibility orlongitudinal stability upon expansion, etc. One pattern may be selectedover another in an effort to optimize those parameters that are ofparticular importance for a particular application.

Referring to FIGS. 1A and 1B, there are provided two exemplary balloonexpandable stent designs. FIG. 1A shows a tubular balloon expandablestent 100 with end markers 103 to increase visibility of the stent 100.The stent 100 is composed of stent struts of a ring 101, ring connectors102, and end markers 103.

Referring to FIG. 1A, the stents 100 are made of multiple circumstantialrings 101, where the ring connectors 102 connect two or three adjacentrings 101 to hold the rings in place. For the end markers 103, FIG. 1 Ashows a “disc” shaped marker. Actually, the shape is not critical solong that the marker can be used to increase further visibility to thestents 100. FIG. 1B shows a tubular balloon expandable stent 104 whichis similar to the stent 100 as shown in FIG. 1A except that the stent104 comprises of center markers 105, 106. The center markers 105, 106help to locate an aneurysm opening during an implantation operation. Thecenter markers 105, 106 can be of the same material and shape as the endmarkers 103.

Referring to FIG. 2, there is provided a self-expanding stent 107 thatis made of wires/ribbons. While a self-expanding stent may have manydesigns, FIG. 2 shows the stent 107 having a typical braided pattern 108with welded ends 109. The stent 107 is so designed that is relativelyflexible along its longitudinal axis to facilitate delivery throughtortuous body lumens, but that is stiff and stable enough radially in anexpanded condition to maintain the patency of a body lumen, such as anartery when implanted therein.

Turning to FIG. 4, it is shown an expanded tubular stent 112. When thetubular stent 112 is fully expanded to its deployed diameter, thelatticework of struts takes on a shape in which adjacent crests undergowide separation, and portions of the struts take on a transverse, almostfully lateral orientation relative to the longitudinal axis of thestent. Such lateral orientation of a plurality of the struts enableseach fully opened cell to contribute to the firm mechanical supportoffered by the stent in its fully deployed condition, to assure a rigidstructure which is highly resistant to recoil of the vessel wallfollowing stent deployment.

While a stent 112 may be deployed by radial expansion under outwardlydirected radial pressure exerted, for example, by active inflation of aballoon of a balloon catheter on which the stent is mounted, the stent112 may be self-expandable. In some instances, passive springcharacteristics of a preformed elastic (i.e., self-opening) stent servethe purpose. The stent is thus expanded to engage the inner lining orinwardly facing surface of the vessel wall with sufficient resilience toallow some contraction but also with sufficient stiffness to largelyresist the natural recoil of the vessel wall.

In one embodiment, the implantable medical devices are intracranialstents 202 and delivery systems for stenotic lesions and aneurysms 201.Due to the characteristics of intracranial blood vessels, theintracranial stents 202 are designed to be very flexible, low profile(0.033″-0.034″ or even less as crimped onto delivery catheter) and thinwall (0.0027″-0.0028″). The intracranial stents 202 do not necessarilyhave the highest possible radial strength because there is no need ofhigh strength for intracranial applications. The radiopacity of theintracranial stents may be provided by either including radiopaquemarkers 205 made from gold or platinum or making the stents 202 fromplatinum/iridium/tungsten alloys. Stents 202 for treating aneurysms 201have a special type of platinum “star markers” 204 in the middle oftheir bodies to assist in precise indication and alignment of the stents202 over the aneurysm neck 201 and allow further operation withaneurysms 201.

As shown in FIG. 3A, the intracranial stent 202 is disposed in thelocation of an aneurysm 201. The membrane 203 partially covers the stent202 and is positioned to seal the neck of the aneurysm 201. Theradiopaque markers 204 are located in the middle of the stent 202 toprovide visibility of the stent 202 during operation and post-operationinspection. Referring to FIG. 3B, a portion of the stent 202 is formedof opened cells 205. This design avoids blocking perforators. Theperforators refer to small capillary vessels that have important anddistinctive blood supply functions. It is possible that tubular stentscan block perforators and inhibit important functions.

Referring to FIG. 4, the delivery system includes a guide wire lumen110, a balloon inflating lumen 111, a connector 116, a balloon cathetershaft 113, and platinum marker bands 115 on the catheter shaft 113. Theguide wire lumen 110 is used for introducing a guide wire in a ballooncatheter, and the balloon inflating lumen 111 for inflating the balloonafter the stent to be placed reaches its targeted location. Theconnector 116 is used for separating the guide wire lumen 110 and theballoon inflating lumen 111. The balloon catheter shaft 113 carries theguide wire lumen 110 and the balloon inflating lumen 111 separately,with a typical length of about 135-170 cm. The ring markers 115 on thecatheter shaft 113 are used for showing the start of balloon tapers andthe edges of the stent. In FIG. 3, an expanded stent 112 is shown beingmounted onto an expanded balloon. The delivery catheter can beessentially a conventional balloon dilatation catheter used forangioplasty procedures. The balloon may be formed of suitable materialssuch as irradiated polyethylene, polyethylene terephthalate,polyvinylchloride, nylon, and copolymer nylons such as Pebax™. Otherpolymers may also be used. In order for the stent to remain in place onthe balloon during delivery to the desired site within an artery, thestent is crimped onto the balloon.

In a preferred embodiment, the delivery of the stent is accomplished inthe following manner. The stent is first mounted onto the inflatableballoon on the distal extremity of the delivery catheter. Stent ismechanically crimped onto the exterior of the folded balloon. Thecatheter/stent assembly is introduced within vasculature through aguiding catheter. A guide wire is disposed across the diseased arterialsection and then the catheter/stent assembly is advanced over a guidewire within the artery until the stent is directly under the diseasedlining. The balloon of the catheter is expanded, expanding the stentagainst the artery. The expanded stent serves to hold open the arteryafter the catheter is withdrawn. Due to the formation of the stent froman elongated tube, the undulating component of the cylindrical elementsof the stent is relatively flat in transverse cross-section, so thatwhen the stent is expanded, the cylindrical elements are pressed intothe wall of the artery and as a result do not interfere with the bloodflow through the artery. The cylindrical elements of the stent which arepressed into the wall of the artery will eventually be covered withendothelial cell layer which further minimizes blood flow interference.Furthermore, the closely spaced cylindrical elements at regularintervals provide uniform support for the wall of the artery, andconsequently are well adopted to tack up and hold in place small flapsor dissections in the wall of the artery.

For resilient or self-expanding prostheses, they can be deployed withoutdilation balloons. Self-expanding stents can be pre-selected accordingto the diameter of the blood vessel or other intended fixation site.While their deployment requires skill in stent positioning, suchdeployment does not require the additional skill of carefully dilatingthe balloon to plastically expand the prosthesis to the appropriatediameter. Further, the self-expanding stent remains at least slightlyelastically compressed after fixation, and thus has a restoring forcewhich facilitates acute fixation. By contrast, a plastically expandedstent must rely on the restoring force of deformed tissue, or on hooks,barbs, or other independent fixation elements.

The presence of a stent in a vessel tends to promote thrombus formationas blood flows through the vessel, which results in an acute blockage.In addition, as the outward facing surface of the stent in contact orengagement with the inner lining of the vessel, tissue irritation canexacerbate restenosis attributable to hyperplasia. Moreover, it isdesirable to deliver drugs or reagents into the aneurysms to enhance theblockage of blood flow into the aneurysms. Finally, implantable medicaldevices have been used as vehicles to deliver drugs or reagents tospecific locations within the vascular system of a human body.

In one example, an intracranial stent 202 is specially designed for lowpressure deployment. The stent 202 has adequate radial strength fortargeting a specific environment of fragile intracranial vessel. Thestent 202 is designed to allow for delivering high stent performance andabsolutely conforming longitudinal flexibility.

Low pressure deployment of a stent is defined as a pressure equal to orbelow 4 atm. This level of pressure enables the stent 202 to be fullydeployed to support a stenosed intracranial vessel or aneurysm neck 201without introducing trauma or rapture of a target vessel. The stent 202can be deployed using balloon techniques or be self-expandable.

The stent 202 comprises structural elements that restrict potential overexpansion, matching the inner diameter of the vessel and to makedeployment extremely precise. This feature of the structural elements incombination with low pressure deployment potentially reduces vesselinjury, rupture or restenosis.

The stent 202 also has longitudinal flexibility equal to or better thanwhat is provided by a delivery catheter. This means that the stent doesnot add increased rigidity to the device. The trackability of the stent202 depends on the mechanical properties of the catheter and is notrestricted by stent 202 alone. The longitudinal flexibility of the stent202 can be measured by force in grams to deflect the stent from neutralline. This force brings stent deflection to 1 mm for less than 8 grams.Existing catheters can provide 20-22 grams per 1 mm deflection. Thiscondition is also extremely important when creating stent compliance toparticular vessels and saves the vessel from possible traumaticreaction.

The structure of the stent 202 is designed to provide a normalizedradial force of 18-19 grams/mm of length and may reach values close tothe ones found in existing coronary stents. Stent structural supportprovides 3-4% of deflection of the stent structure together withintracranial vessel wall natural pulsing. This leads to greater stentconformity and a reduced vessel injury score.

-   -   The intracranial stent 202 has profile in compressed delivery        mode 0.020″.    -   The intracranial stent 202 is designed to be compressed onto        delivery catheter with a profile as low 0.014″-0.016″ having        stent profile 0.020″-0.022″.    -   The intracranial stent 202 has even material distribution and        wall coverage, creating needed vessel support. The material        ratio is in the range of 10-17% depending on deployment        diameter.    -   The intracranial stent 202 has a strut thickness and width not        larger than 0.0028″.

Strut dimensions are selected which make the least intrusive stentmaterial volume and to reduce the vessel injury score.

The stent surface to length ratio is set to be 1.1-1.3 mm²/mm to provideminimal vessel injury score.

At least one membrane 203 is disposed onto the outer surface of a stent202. The membrane 203 comprises pockets which serve as receptacles fordrugs or reagents to deliver the drugs or reagents into vascularsystems. The membrane 203 covers a part of a stent 202 as shown in FIGS.3A and 3B, wherein the size of the membrane 203 is variable depending onapplication. In one example, the membrane 203 covers the whole outersurface of a stent 202. Thus, the membrane 203 may be in any shape orsize.

In certain embodiments, the membrane 203 comprises a first layerattached to the outer surface of an implantable medical device such as astent 202. An intermediate layer is attached to the first layer whereinthe intermediate layer comprises at least two circumferential stripsbeing separated from each other and a second layer covering the firstlayer and the intermediate layer. The spaces surrounded by the firstlayer, the circumferential strips and the second layer form the pocketsthat serve as receptacles for drugs or reagents. In other embodiments,the intermediate layer includes at least one opening so that the pocketscan be formed within the openings. The shapes and sizes of the openingsmay vary in accordance with specific applications. As shown in FIG. 5, astent 202 is partially covered by a membrane 203 that comprises a firstlayer 206 and a second layer 207. FIG. 5 also shows the drug releasingpores 208.

Many polymeric materials are suitable for making the layers of themembrane 203. Typically, one first layer is disposed onto the outersurface of a stent. The first layer has a thickness of 0.002″-0.005″with pore sizes of 20-30 microns and similar to nominal initialdiameter.

In certain embodiments, the first layer serves as an independentmembrane 203 to mechanically cover and seal aneurysms 201. In certainembodiments, the first and/or second layers can be comprised ofbiodegradable material as a drug or reagent carrier for sustainedrelease.

It is desirable that the intermediate layer be formed of a materialwhich can fuse to the first and second layers or attached to the firstlayer in a different manner. In certain embodiments, the intermediatelayer may be merged with the first layer to form a single layer withrecessions within the outer surface of the merged layer.

The second and intermediate layers can be made of biodegradable materialthat contains drugs or reagents for immediate or sustained controlledrelease. After biodegradable material is gone through the degradationprocess, the membrane 203 is still in tact providing vessel support.

The second layer may be composed of a polymeric material. In preferredembodiments, the second layer has a preferable thickness of about 0.001″with pore sizes of about 70-100 microns.

The polymeric layers may also be formed from a material selected fromthe group consisting of fluoropolymers, polyimides, silicones,polyurethanes, polyurethanes ethers, polyurethane esters,polyurethaneureas and mixtures and copolymers thereof. Biodegradablepolymeric materials can also be used.

The fusible polymeric layers may be bonded by adhering, laminating, orsuturing. The fusion of the polymeric layers may be achieved by varioustechniques such as heat-sealing, solvent bonding, adhesive bonding oruse of coatings.

Types of drugs or reagents that may prove beneficial include substancesthat reduce the thrombogenic, inflammatory or smooth muscle cellproliferative response of the vessel to the implantable medical devices.For example, cell inhibitors can be delivered in order to inhibit smoothmuscle cells proliferation. In intracranial or some other applicationsfibrin sealants can be used and delivered to seal aneurysm neck andprovide fibroblasts and endothelial cells growth. Specific examples ofdrugs or reagents may include heparin, phosporylcholine, albumin,dexamethasone, paclitaxel and vascular endothelial growth factor (VEGF).

The drug or reagents can be incorporated into the implantable medicaldevices in various ways. For example the drug or reagent can be injectedin the form of a gel, liquid or powder into receptacles of the pockets.Alternatively the drug or reagent can be supplied in a powder which hasbeen formed into a solid tablet positioned in the receptacles.

Another prerequisite of a successful treatment of these extremely smalldiameter vessels is that the stent delivery system is highly flexible toallow it to be advanced along the anatomy of the cerebral circulation.In addition, the total stent delivery system must be of extremely smallprofile, to treat diseased intra-cranial arteries generally ranging from1.5 mm to 5 mm.

Referring to FIG. 6, in certain embodiments a membrane 203 is embodiedas a sleeve 301 supported by two ring-like short stents 302 at both endsof a device so that the membrane 203 covers the whole area of the device302. There is no scaffold support in the middle of the device 302.Radiopaque markers 303 are located at both ends of the stent 302.Depending on applications, the rings are balloon expandable and madefrom stainless steel or self-expandable made from NiTi (memory shapednickel-titanium alloy).

The membrane 203 is part of a hemorrhagic stent structure designed toeffectively occlude aneurysm neck and “recanalize” the vessel. It'llallow rebuilding vessel and essentially eliminating aneurysm. No need ofexpensive (and extra-traumatic, sometimes too massive) coiling isexpected.

This device is a preferable solution to treat: giant and wide neckaneurysms, bifurcation and trifurcation aneurysms. It is also apreferred treatment solution for cc fistula ruptured in cavernous sinus,pseudoaneurysms, saccular aneurysms.

The membrane 203 is elastic to allow its own expansion five to six timeswithout disintegration and detachment from the stent structure. Thethickness of the membrane 203 is expected to be not more than 0.002″ incrimped position and 0.001″ in expanded form. The mechanical propertiesdo not introduce extra rigidity to the intracranial stent 202 and haveno resistance to stent expansion. The membrane material also allows anexpanded membrane 203 to endure normal blood pressure.

The membrane 203 is not solid, but is formed as strips between stentstruts, or with a series of holes or ovals. The membrane 203 thereforecould be porous, or woven mesh. The membrane 203 could also be designedand structured in a way such that there is a system of holes to allowblood penetration into the system of perforators and not allow it intothe aneurysm 201.

For upper brain arteries above Siphon, a porous and permeable membrane203 is ideal. Such a membrane 203 treat an aneurysm neck 201 withoutblocking microvessels (perforators). It is expected that interventionalneuroradiologists (INRs) to be more willing to use the membrane 203 thanother known techniques for dealing with aneurysm necks 201. Thepermeable membrane 203 has a system of holes or pores with bordersbetween them not larger than 100 microns. The holes or pores may rangebetween 50 to 100 microns. The membrane 203 is able to significantlyimprove hemodynamics around the aneurysm 201, since it has a lowerdelivery profile and is more flexible compared to a stent 202 with asolid membrane.

The membrane 203 is attached to the stent struts. The membrane 203 maybe attached using spraying, a dipping technique or heat bonding to theintermediate polymeric layer. The stent 202 is placed on a mandrel (hardPTFE or metal), or hung on a hook and the PU solution is sprayed andsolidified with a quick drying process. Alternatively, the stent 202 isplaced on the mandrel or on the hook and submerged into a PU solution.

A biodegradable membrane 203 enables drug delivery and is laterdissolved. There are applications where there is no need for a membrane203 to exist after exceeding 15 to 20 days after placement and thus themembrane 203 could be dissolved.

The membrane 203 may be made from PU, Silicon, or any other elastomericmedical grade polymer.

Referring to FIG. 7, a membrane 203 for bifurcational stents 202 totreat a bifurcation or trifurcation aneurysm 201 is provided. At least30 to 35% of aneurysms are located at bifurcation sites of intracranialvessels. This membrane 203 is one-sided and non-circumferential. Thebifurcation stents 202 are joined by a membrane 203 to cover theaneurysm neck 201. The same pattern can be applicable to self-expandable(super-elastic) or balloon expandable (stainless steel, CoCr, Ptlralloys) stents 202.

Referring to FIG. 8, an aneurysm 201 is covered with the membrane 203 ofan intracranial stent 202 to treat and prevent ischemic and hemorrhagicstroke. The intracranial stent 202 coupled with a membrane 203 acts as ascaffold to open clogged arteries, and as a cover to prevent bloodcirculation to the aneurysm 201. Obstructing blood supply to theaneurysm 201 isolates the aneurysm 201 from normal blood circulation,and thereby eventually causes it to dry out. Complete obstruction to theaneurysm 201 may not be necessary.

FIG. 9 provides a table with typical dimensions for the intracranialstent 202 for use with the membrane 203. The material for the membrane203 is biocompatible, has good adhesion to stent struts made fromstainless steel 316L, and is formed by a stable film. In otherembodiments, the film is blood “permeable” rather than being a solidfilm. The covered sections, that is, the borders between pores or holesdo not exceed 75 μm so as to prevent any part of the stent 202 or themembrane 203 from blocking perforators. Several options can beundertaken to achieve this. The membrane 203 is made from a thin filmthat does not exceed 0.001″ in width. The film has good expandability,and can expand up to 400% at a low force. The membrane 203 also has ashelf life or chemical stability at ambient conditions and is stable insterilization conditions (Eto).

In one example, polyurethane is used to make the membrane 203.Specifically, solution grade aromatic, polycarbonate based polyurethaneis used. The physical properties are: durometer (Shore) is 75A, tensilestrength is 7500 psi and elongation to 500%.

Referring to FIG. 10, to make a permeable membrane 203, holes aredrilled into a solid film to form pores. The pore size is between 0.025to 0.050 mm, while the distance between pores is less than 100 μm.

Referring to FIG. 11, threading strips 203 of a polymer are wrappedlaterally around the stent 202. The strips are interlaced above andbelow the struts of the stent. The width of the strips is less than0.075 mm and distance between adjacent strips is less than 100 μm.

Referring to FIG. 12, a sheet of weaved material 203 is wrapped aroundthe stent 202. The mesh size of the sheet is around 0.025-0.050 mm,while the width of the polymer is less than 100 μm.

Referring to FIG. 13, the film 203 completely surrounds the stent strutand is a stable film between the struts of the stent. The film betweenstruts is either within the struts or on the outer struts. The polymericfilm stays as close to vessel wall as possible. This is to minimize thefilm “flipping” inside of vessel as shown in FIG. 16.

Referring to FIG. 14, the membrane 203 is secured onto the struts, andis difficult to dislodge or be torn from the stent 202. The thickness ofthe membrane 203 does not add any significant profile to the crimpedassembly, that is, it contributes to less than 0.001″ of the crimpedstent profile. The membrane 203 also has uniform shrinkability.

Referring to FIG. 15, the membrane 203 may completely cover the stent202, cover the mid-section of the stent 202, or cover a radial sectionof the stent 202. The membrane 203 expands with the stent 202 and doesnot restrict or alter the expansion characteristics of the stent 202.The membrane 203 is easily expandable up to 400%. The membrane 203 has aminimum effect on the mechanical properties of the stent 202 such asflexibility, trackability, expandability, recoil and shortening. Themembrane 203 is also stable in normal shelf life conditions and stablein sterilization conditions (Eto). The properties of the polymer filmare preserved and not changed after sterilization. The membrane 203 isprevented from sticking to the balloon material (Nylon) after crimping.The membrane 203 is able to tolerate temperature variations (of up to60C). The edges of the membrane 203 are aesthetically acceptable, andhave smooth, not rough edges.

Referring to FIGS. 17 to 19, the stent 202 is used to treat abifurcation or trifurcation aneurysm 201. The stent 202 is implanted tobe partially located in a main artery extending to be partially locatedin a subordinate artery. For example, in FIG. 17, two vertebral arteriesjoin to the basilar artery. The stent 202 is deployed such that it islocated in the basilar artery and in a vertebral artery (right side)where the aneurysm 201 is formed. On the other vertebral artery (leftside), blood continues to flow to the basilar artery without anyobstruction since the membrane 203 is permeable to blood flow.Preferably, the membrane 203 covers the whole stent 202, and thepermeability of the membrane 203 allows blood flow through the leftvertebral artery (left side).

In FIG. 18, the middle cerebral artery divides into the superior trunkand the inferior trunk. The stent 202 is deployed such that it islocated in the middle cerebral artery and in the inferior trunk. Again,the struts of the stent 202 do not inhibit blood flow to the superiortrunk, and blood flows through the stent 202 to the inferior trunk.

In FIG. 19, the stent 202 is deployed in the vertebral artery. As theaneurysm 201 in this example is located in a middle portion of thevertebral artery, there is no need for the stent 202 to be located inmore than one artery.

When implanted, the stent 202 diverts blood flow away from the aneurysm201. This leads to occlusion of the aneurysm 201 and keeps the arterialbranches and the perforators patent. The stent 202 does not requireprecise positioning because preferably, it is uniformly covered with thepermeable membrane 203. In other words, most of the circumferentialsurface of the stent 202 is covered by the membrane 203. Due to theparticular porosity and dimensions of the membrane 203, bloodcirculation to the aneurysm 201 is obstructed while blood supply toperforators and microscopic branches of main brain arteries as well aslarger arteries is permitted. As described earlier, obstructing bloodsupply to the aneurysm 201 isolates the aneurysm 201 from normal bloodcirculation, and thereby eventually causes it to dry out. The stent 202and membrane 203 treats the aneurysm 201 by causing an alteration in thehemodynamics in the aneurysm sac such that intra-aneurysmal thrombosisis initiated. At the same, blood flow into the arteries (branch, main,big or small) are not significantly affected by the implantation of thestent 202 or the membrane 203 due to the special porosity of themembrane 203.

Although a bifurcation aneurysm has been described, it is envisaged thatthe stent 202 may be used to treat a trifurcation aneurysm in a similarmanner.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention as shown inthe specific embodiments without departing from the scope or spirit ofthe invention as broadly described. The present embodiments are,therefore, to be considered in all respects illustrative and notrestrictive.

1. A method for treating a bifurcation or trifurcation aneurysmoccurring on a first artery, the first artery and a second arteryjoining to a third artery, the method comprising: inserting a medicaldevice such that it is at least partially located in the first arteryand is at least partially located in the third artery; expanding themedical device from a first position to a second position, said medicaldevice is expanded radially outwardly to the second position such thatthe exterior surface of said medical device engages with the innersurface of the first and third arteries so as to maintain a fluidpathway through said arteries; and positioning the medical device suchthat a membrane of the medical device is located against an aneurysmneck of the aneurysm to obstruct blood circulation to the aneurysm whenthe medical device is expanded to the second position, and at least aportion of the membrane is secured to the medical device to maintain theposition of the membrane relative to the medical device when expanded tothe second position; wherein the membrane is permeable and porous, thesize of the pores of the membrane and the ratio of the material surfacearea of the membrane being such that blood supply to perforators and/ormicroscopic branches of main brain arteries is permitted to improvehealing of the first artery but blood supply to the aneurysm isprevented. 2-31. (canceled)